line height vs. margins

Is there any consensus on where the extra space from line-height
should be rendered?

Let's say I have some text which paginates as follows.
Let x be the space between paragraphs, and y be the space
between lines.

previous paragraph
                    <- x
line1 line1 line1 /- y
line2 line2 line2
                    <- x
next paragraph

Now, let's suppose we set the line-height of the middle paragraph
to be 10 pixels more than what the auto line-height value was.

previous paragraph

                    <- x'
line1 line1 line1
                    <- y'
line2 line2 line2

                    <- x''
next paragraph

We can safely assume that y' = y + 10px, but what about x' and x''?
Netscape seems to stick all the whitespace above the text, so that
x' = x+10px, and x'' = x.  Microsoft IE seems to do something
different, but I can't tell what it is. Perhaps x'=x+5px and x"=x+5px?

What *should* it do? On the one hand, we could leave it up to
the UA, but if the point of CSS is to allow the web author to
specify layout accurately, it seems that this would be a good
thing to nail down.